Stable partial nitritation for low-strength wastewater at low temperature in an aerobic granular reactor

Partial nitritation for a low-strength wastewater at low temperature was stably achieved in an aerobic granular reactor. A bench-scale granular sludge bioreactor was operated in continuous mode treating an influent of 70 mg N-NH4+ L-1 to mimic pretreated municipal nitrogenous wastewater and the temperature was progressively decreased from 30 to 12.5 degrees C. A suitable effluent nitrite to ammonium concentrations ratio to a subsequent anammox reactor was maintained stable during 300 days at 12.5 degrees C. The average applied nitrogen loading rate at 12.5 degrees C was 0.7 +/- 0.3 g N L-1 d(-1), with an effluent nitrate concentration of only 2.5 +/- 0.7 mg N-NO3- L-1. The biomass fraction of nitrite-oxidizing bacteria (NOB) in the granular sludge decreased from 19% to only 1% in 6 months of reactor operation at 12.5 degrees C. Nitrobacter spp. where found as the dominant NOB population, whereas Nitrospira spp. were not detected. Simulations indicated that: (i) NOB would only be effectively repressed when their oxygen half-saturation coefficient was higher than that of ammonia-oxidizing bacteria; and (ii) a lower specific growth rate of NOB was maintained at any point in the biofilm (even at 12.5 degrees C) due to the bulk ammonium concentration imposed through the control strategy. (C) 2015 Elsevier Ltd. All rights reserved.